Ten airline employees at a Texas airport have been indicted for their role in allegedly trafficking nearly 150 pounds of what the defendants believed was methamphetamine.

Nine of the 10 employees have been arrested in the undercover operation at Dallas/Ft. Worth International Airport, FBI officials said at a press conference in Texas on Tuesday. One had not yet been apprehended.

Officials would not say what prompted the sting, but an indictment filed in federal court indicates the investigation goes back as far as August 2016.

Officials at the press conference and publicly available court documents did not detail how exactly the defendants used their employment at the airport to smuggle contraband, but they accused them of detailing to undercover officers how they evaded security and distributed a substance that the workers believed to be methamphetamine to multiple cities, including Newark, Charlotte and Phoenix on commercial flights.

On each occasion, law enforcement officers received the counterfeit drugs at the flight's destination.

“At American and Envoy Air, we have an unwavering commitment to the safety and security of our customers and team members," American Airlines, which owns Envoy Air, told ABC News in a statement. "We take this matter very seriously and are cooperating with law enforcement during their investigation.”

The Envoy employees in the indictment have been suspended, ABC News has learned.

Spirit Airlines told ABC News "Spirit has been advised of this matter and will cooperate fully with law enforcement."

During the investigation, defendant Pabon allegedly told undercover officers his organization could transport guns and explosives, including C4, via commercial airlines. According to court documents, discussions about transporting C4 also took place.

Those arrested are expected to appear in U.S. federal court later this week and face charges of conspiracy to possess with intent to distribute a schedule II controlled substance. They have not yet entered pleas.

All employees at major airports undergo background checks and the screening of both passengers and airline employees is handled by the Transportation Security Administration.

Editor's note: The top line of this story has been updated to reflect that the nearly 150 pounds was what the defendants thought was methamphetamine, not what federal officials thought was methamphetamine.